Abstract: 7050-T7451 aluminum alloy was welded in friction stir welding at different welding parameters.The structure of the joints was studied with optical microscopy and TEM.Tensile strength and hardness distribution of the joints were tested.The mechanical properties, the microstructure and welding defects of the joints were controlled by changing welding parameters.The tensile strength of the joint was up to 88% of that of base metal at the parameters with tool traverse speed of 200 mm/min and rotation rate of 800 r/min.The fracture location was in HAZ in the joints welded in higher heat input;when the heat input was lower, kissing bond was produced in the root of the joint, and then fracture initiated from the location of the defect stretched.The result showed that dynamic recrystallization occurred in nugget zone together with the precipitates dissolved.Precipitates were coarsened in HAZ accompanying with the presence of precipitate-free zone.